The concept of using orthogonal sub-channels (OSC), also referred to as Multiple Users Reusing One Timeslot (MUROS), for doubling voice capacity was previously introduced. The OSC concept allows the network to multiplex two wireless transmit/receive units (WTRUs) that are allocated the same radio resource. The sub-channels are separated by using non-correlated training sequences. The first sub-channel can use existing training sequences, and the second sub-channel can use new training sequences for both the downlink and the uplink. Alternatively, only new training sequences or only existing training sequences can be used on the sub-channels. Using OSC can double voice capacity with negligible impact to WTRUs and networks. OSC can transparently be applied for all Gaussian minimum shift keying (GMSK) modulated traffic channels (e.g., for full rate traffic channels (TCH/F), half rate traffic channels (TCH/H), a related slow associated control channel (SACCH), and a fast associated control channel (FACCH)).
One current goal with MUROS is to increase the voice capacity of the system. For example, the voice capacity can be increased by having two circuit switched voice channels (i.e., two separate calls) on the same radio resource. By changing the modulation of the signal from GMSK to QPSK (where one symbol maps to two bits), it is relatively easy to separate two users—one user on the X axis of the constellation and a second user on the Y axis of the constellation. The network sends only one signal, but it contains information for two different sub-channels (users).
In the downlink, the OSC concept can be realized in the transmitter of a base station (BS) using a quadrature phase shift keying (QPSK) constellation that may be, e.g., a subset of an 8-PSK constellation used for enhanced general packet radio service (EGPRS). Modulating bits are mapped to QPSK symbols (“dibits”) so that the first sub-channel (OSC-0) is mapped to the most significant bit (MSB) and the second sub-channel (OSC-1) is mapped to the least significant bit (LSB). Both sub-channels may use individual ciphering algorithms, e.g., A5/1 or A5/3. Several options for symbol rotation may be considered and optimized by different criteria. For instance, a symbol rotation of 3π/8 would be as in EGPRS, a symbol rotation of π/4 would make it like π/4-QPSK, and a symbol rotation of π/2 can provide sub-channels to imitate GMSK. Alternatively, the QPSK signal constellation can be designed so that it appears like a legacy GMSK modulated symbol sequence on at least one sub-channel, e.g., it is legacy compliant.
Another method to realize the OSC concept in the downlink is to multiplex two WTRUs together by transmitting two individual GMSK modulated bursts per timeslot. Interference-cancellation type receivers can be used for reasonable demodulation performance in the presence of the other multiplexed user. It is not precluded that at least one multiplexed user employs a conventional type of equalizer receiver.
In the uplink, each WTRU can use a normal GMSK transmitter with an appropriate training sequence. The BS typically employs interference cancellation or joint detection type of receivers, such as a space time interference rejection combining (STIRC) receiver or a successive interference cancellation (SIC) receiver, to receive the orthogonal sub-channels used by different WTRUs.
Typically, during the OSC mode of operation, the BS applies downlink and uplink power control with a dynamic channel allocation (DCA) scheme to keep the difference of received downlink and/or uplink signal levels of co-assigned sub-channels within, e.g., a ±10 dB window, although the targeted value may depend on the type of receivers multiplexed together and other criteria.
The basic OSC or MUROS concept may or may not be operated in conjunction with Frequency-Hopping or User Diversity schemes, either in the DL, in the UL, or both. For example, on a per-frame basis, the sub-channels may be allocated to different pairings of users, and pairings on a per-timeslot basis may be recurring in patterns over prolonged period of times, such as several frame periods or block periods. The ideas presented herein apply equally to these modifications of the baseline OSC or MUROS concepts.
The OSC or MUROS concept has been proposed to increase voice capacity in a GSM system. However, while voice is an important multiplexing case, GSM/EGPRS systems in practice also rely on more sophisticated service multiplexing scenarios, such as packet switched (PS) services through GPRS/EGPRS, simultaneous support of voice and data through DTM, and so on. Unless the MUROS concept can be extended to also allow for operation in these additional service scenarios, its benefits are confined to voice channel multiplexing only. Therefore, it would be desirable to explore other advantageous applications of the OSC concept.